Raster scan display devices may be divided into two general groups. The first of these groups is the character generator system in which a character set for display is held in a store and this store is accessed at locations each of which corresponds to one character in the set.
One example of such a system can be seen in U.S. Pat. No. 3,543,244 (Cuccio). In this system, a display controller forms a part of a data communication system and controls the display of incoming data on a plurality of displays. Input data is stored in a memory and is read therefrom to generate addresses of a character generator. The character generator, under the control of a display timing circuit, produces individual character data in response to the addresses from the memory, and applies this data to a video distributor for display on one or more display devices.
In U.S. Pat. No. 3,614,766 (Kievit), a character generator is accessed by a Y matrix counter and an X matrix counter to produce character data for display on a T.V. monitor. Prior to transmission to the monitor, the character data is mixed with color data from a core memory to produce composite display signals.
U.S. Pat. No. 4,068,225 (Lee) shows another character generator system in which character data for display is held in a memory in ASCII code form and read out to a character generator to produce display dot patterns. The generator output is applied to a video register in byte form and serially shifted therefrom in response to signals from a video dot counter.
U.S. Pat. No. 4,117,469 (Levine) shows a display system coupled to a microprocessor. Here again coded character data from a memory drives a character generator to generate video display signals.
Lastly, in U.S. Pat. No. 4,309,700 (Kraemer) video signals in a CRT display system are generated by addressing a character generator in the form of a read-only memory.
The character generator system has the great advantage that it is efficient in the use of memory space. Thus, for example, a character `A` dot pattern is held in the character generator only once irrespective of the number of times it is used in a full screen of displayed characters. It is, therefore, of particular value for alphanumeric displays. It can also be employed for graphics displays by generating, as characters, portions of lines, straight or curved, to be displayed. Thus, a graphics picture can be built up by the use of successive line `characters` which join together to provide the required graphic picture. However, this use is limited, especially for high resolution graphics displays, by the need to alter the character generator data frequently in order to accomodate the almost infinite number of curves and angled lines which can be generated and may be required.
In order to overcome this problem, the systems of the second general group were developed. These are the bit-mapped raster graphics systems. In these systems, a pattern of data corresponding to the pattern of dots to be displayed by the display device, is stored. All that is then required is sequential access of the store to read out the dot pattern for display. This system, though relatively expensive in terms of storage capacity requirements, has the advantages that any required dot pattern can be stored and either portions of the pattern or the complete pattern can be rapidly changed.
An example of a bit-mapped raster display system can be seen in an article entitled `Computer Graphics in Color` by P. B. Denes, which appeared in the Bell Laboratories Record, May 1974 at pages 139 through 146. This uses a memory storing the codes for successive points on the display device in successive memory locations. These codes each comprise three bits which define color information.
U.S. Pat. No. 4,070,710 (Sukonick) shows a system in which a bit mapped memory stores successive points for display. These points are, in fact, more than can be displayed at any one time, so, by selecting different initial addresses in the memory, different displays can be obtained without altering the stored data. Thus, the displayed picture can be panned, both horizontally and vertically, or a split screen display, using data from different portions of the memory, can be created.
Lastly, U.S. Pat. No. 4,149,152 (Russo) shows a bit mapped system which includes an auxilliary memory in addition to the bit mapped memory. The auxilliary memory, which is smaller than the bit mapped memory, stores data specifying dot colors of contiguous dot elements on the display.